The present invention relates to improved lubricants for spacecraft.
The use of satellites for communication and navigation is ever increasing in both military and commercial applications. The high costs of building and launching satellites are driving the need to extend the useful life of satellites from the current 5 to 8 years to at least 15 years.
Spacecraft utilize many moving assemblies. The current tribological requirements of such assemblies are usually satisfied by a variety of lubricants and materials. To date, spacecraft lifetime is limited primarily by the failure of systems such as power supplies, electronics, thermal systems, optical systems and positioning systems. Technological advances in these systems are making them more reliable. As spacecraft life expectancy increases, more spacecraft failures will be attributed to tribological limitations if corresponding advances in tribology do not occur.
Lubrication demands on satellite platforms generally fall into three categories: high speed, low speed and mixed speed. Some manufacturers of mechanisms on satellite platforms prefer liquid lubricants while others prefer grease lubricants. Low speed satellite mechanisms operate below the speeds required to produce an elastohydrodynamic lubrication (EHL) film, and thus have metal to metal contact. Such metal to metal contact leads to high wear and eventual mechanism failure. High speed mechanisms operate at speeds where the EHL film is maintained throughout the life of the bearing system. Although the presence of the EHL film minimizes wear, there is still intermittent asperity contact at full speed and high wear during start-up. Mixed speed mechanisms operate at times at high speed and at other times low speed, and are exposed to both EHL and boundary lubrication.
Two factors are critical in maintaining good lubrication in liquid/grease lubricated systems over an extended time, in an extremely high vacuum. First, the lubricant base oil must remain in place, without volatilizing or creeping into other areas, and it must not change in other ways, such as becoming thicker or changing chemically. Second, additives in the lubricant must not evaporate or be consumed, thus leaving the base oil to carry the load with no additive-produced film. New, improved base fluids for satellite applications are much less volatile than previously and currently used mineral oils; these new base fluids, including, but not necessarily limited to narrow molecular weight range polyalphaolefins (PAO), multiply alkylated cyclopentanes (MAC) and silahydrocarbons (SiHC), are gradually being inserted into satellite applications.
Additives also need to have low volatility. Hydrocarbon base lubricants are readily enhanced with a wide variety of additive chemical classes. In atmospheric pressure applications, commercial additives are a mature technology because hydrocarbon base oils have a very large industrial market. One problem for high vacuum applications is that commercial additives are often supplied in a carrier fluid, such as a mineral oil or an ester oil, which is more volatile than the additive and therefore undesirable for satellite applications. Further, most commercial additives are not made especially for vacuum operation, so the choice is limited. Commercial additive producers have little incentive to make less volatile additives for the satellite lubricant market because of its extremely small volume.
U.S. Pat. No. 5,196,130, issued Mar. 23, 1993 to L. J. Gschwender and C. E. Snyder, Jr, discloses a lubricity additive, tris(4-chlorophenoxyphenyl)phosphate, for high-temperature gas turbine engine oils. We have now found that this additive is also useful for satellite lubrication applications.
Accordingly, it is an object of the present invention to provide lubricants for satellite applications.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.